Category: Solar Energy (Photovoltaics)
Category: Photovoltaics (IEEE 1547)
Aim: Enabling high penetration of large-scale PV by reducing interconnection cost and time
Method: DER installed on distribution feeders can operate reliably while connected to the utility. But when this connection is broken, the circuit may remain energized by the remaining DER. IEEE 1547 requires DER to disconnect from such islands within 2 seconds. But this becomes difficult when a wide DER variety (manufacturers of various DER types employing different detection methods at multiple physical locations) is spread across the island.
Therefore, large scale DER installations may need an overarching communications-based protection system. The most common is Direct Transfer Trip (DTT). In this system, a central controller monitors feeder midline reclosers and then sends trip signals to appropriate disconnecting devices.
A new DTT alternative is the subject of this presentation. Named “Distributed Generation Permissive” (DGP), this new method continuously monitors the continuity of the connection between utility and DER. DGP uses power line communications to continuously send a multi-frequency digital message across this path. Loss of this message means the path is broken and an island has likely formed. DERs will then be automatically tripped, based on the message no longer getting through.
Results: DGP has been installed on dozens of distribution circuits. It has been used to protect DER of many types, including solar PV and energy storage. Accumulated experience now exceeds 5 years and overall reliability after installation and commissioning has been excellent.
Of particular relevance is a 23KV distribution circuit with 10 grid-attached PV sites that employ DGP to prevent islanding. This circuit nicely illustrates the high penetration scenario that is becoming increasingly common. The DGP system has now been in service for more than a year and is proving to be both technically and economically superior to DTT.
The DGP system offers the flexibility of attaching additional new DER sites to a “DG Ready” circuit anywhere along the feeder at any given time (i.e. as construction is completed and the DG is ready to connect), and the scalability of adding new DER sites with minimal effort and low incremental cost.
Conclusions: The utility found this solution to be less costly than DTT systems employing other communications media. They have found DGP to be both dependable and secure, as they reported in a 2018 T&D World article. With DER penetration predicted to grow rapidly, the continued use of traditional AI methods must now be questioned. DGP offers a proven, more economical and reliable method to deliver AI protection and enable large scale deployments. The DGP platform can also be used to support new modes of operation with smart inverters as required by the new 1547-2018 standard by sending control signals from the utility to DER.
Nachum Sadan– President & CEO, GridEdge Networks